[TEST] Adding Sparse Triangle to Unittest

[henrydingliu]

Summary of Changes

Fixing test fixture to supply actual sparse triangles
sped up numpy conversion of prism in test_triangle
resolved failing tests in test_development
added numpy conversion in learning to satisfy skl requirement that sample_weight is dense
resolved failing test in cape_cod
added backend conversion in mack for sparse compatibility
added nanquantile and nanmedian to sparse
added new tests for nanquantile
added nan to nan_to_num in sparse

Related GitHub Issue(s)

fixes #1155

Additional Context for Reviewers

• I passed tests locally for both code (uv run pytest) and documentation changes (uv run jb build docs --builder=custom --custom-builder=doctest)

---

Note

Medium Risk
Changes Mack reserve uncertainty recursion and ML sample-weight preparation alongside new sparse reduction helpers; scope is moderate but touches numerically sensitive reserving code paths.

Overview
This PR fixes sparse triangle unittest coverage (#1155) by yielding real sparse-backed triangles from fixtures via set_backend instead of toggling global ARRAY_BACKEND, and runs large prism tests on sparse-only (with a trimmed prism_convert fixture for auto-sparse checks).

Sparse utilities gain nanquantile and nanmedian for COO arrays, plus a configurable nan fill in nan_to_num, with new unit tests.

Sparse compatibility fixes in MackChainladder align triangle backends before concatenation and Mack recursion (e.g. after sum triggers auto-sparse), and DevelopmentML._prep_w_ml forces dense numpy/cupy weights for scikit-learn.

Tests are updated to use Triangle equality and numpy assertions where sparse paths differ, and CapeCod uses at for sparse assignment.

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[henrydingliu]

@genedan can you take care of the 6 failing tests in test_slicing?

[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.1% of exported symbols fully typed (195 / 1294)

	Known	Ambiguous	Unknown	Total
Project (head)	195	110	989	1294

Other symbols referenced but not exported by chainladder: 13

	Known	Ambiguous	Unknown	Total
Other (head)	3	1	9	13

Symbols without documentation:

• Functions without docstring: 315
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 28.6% fully typed (2 / 7); 2 no longer exported

	Known	Ambiguous	Unknown	Total
Patch	2	0	5	7

Patch symbol details

Symbol	Status	Change
chainladder.utils.sparse.nanmedian	❌ unknown	new
chainladder.utils.sparse.nanquantile	❌ unknown	new
chainladder.utils.tests.test_sparse.test_1D_nanquantile	✅ known	new
chainladder.utils.tests.test_sparse.test_keepdims_nanquantile	✅ known	new
chainladder.utils.tests.test_utilities.test_maximum_2	❌ unknown	new
chainladder.utils.tests.test_utilities.test_maximum_minimum_1	❌ unknown	new
chainladder.utils.tests.test_utilities.test_minimum_2	❌ unknown	new
chainladder.utils.tests.test_utilities.test_maximum	—	no longer exported (was ❌ unknown)
chainladder.utils.tests.test_utilities.test_minimum	—	no longer exported (was ❌ unknown)

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[henrydingliu]

@genedan there are two more tests in test_triangle that you authored. i'm not sure if those tests should be sparse-compatible. could you please take a look?